A Geology Field Trip

Readings and Notes An Introduction to Earth Science 2016 A Geology Field Trip John J Renton Thomas Repine Follow this and additional works at: https://researchrepository.wvu.edu/earthscience_readings Part of the Geology Commons A GEOLOGY FIELD TRIP by Dr John J Renton Dept of Geology and Geography West Virginia University _,- ~-?IJ~'I Introduction to Field Trip I I I I ~ ~ !3' /f, t/1 l'«( ,,}J,I ,~ I I I I ~ I , J I ~ I ,, ,~ - J ~' I '~, G> ROAD LOG I I " I had several objectives in mind when I prepared this field trip I want to develop in you an interest, an understanding, and an appreciation of geology of the region I want to show you differences between three of the basic physiographic provinces within Appalachia, the Low Plateau, the High Plateau, and the Appalachian Mountain Section of the Valley and Ridge Province I also want to show you the role that the kinds and structures of the underlying rocks plays in the formation of the topography of a region in order to have you better understand why the appearance of the land changes as you travel about FIGURE the country During the trip, you will also see excellent examples of the process of weathering as well as the erosive power of streams The trip will visit I " several tourist areas and, in addition to ::: r their scenic beauty, you will learn that g> they represent excellent examples of 'S important geologic features C, I ~ Cl> ch Ct FIGURE 1* Introduction: As you begin the trip, there are two figures you will want to keep handy; Figure 1*, which is the road map showing the path of the trip and Figure which is the stratigraphic column that lists all of the rock formations you will encounter on the trip and their Is: ::::> :iii: u C! ~ !' _ , , • • ~~ Qr -11 FIGURE Generalized Geologic Map ~l2t:i Appalachian Mountain Section: The Appalachian Mountain Section of the Valley and Ridge Physiographic Province extends from zo~~ the Allegheny Structural Front to the Great Valley and constitutes what is commonly referred to as the Appalachian Mountains The name reflects the dominance of northeast-southwest-trending parallel valleys and ridges within the province For the most part, the ridges are high amplitude asymmetric to overturned folds, commonly broken on the western limb by high-angle thrust faults (Figure 14) The oversteepening of the folds to the west indicates an east to west direction of rock transport during the original mountain building episode The wavelength of the folds in the Valley and Ridge (the distance between fold axes) is significantly less than the wavelength of the more open folds in the High Plateau to the west Many of the anticlinal structures have been breached by erosion, exposing the oldest rocks of the region, the Ordovician, within anticlinal valleys An excellent example of a valley that formed as the result of the breaching of an anticline that you may be familiar with is Germany Valley located in Pendleton County, West Virginia (Figure 15) There are occasional synclinal ridges that you will observe along your route, the most well known being Figure 14 ::r::.e=rAct \ M ~N T' Figure 16a Sc.~ ~'!"\"- O f'&° ,e>U~O~ Sideling Hill, Maryland Synclinal ridges are formed as adjacent anticlines are breached with the subsequently valleys eventually being eroded below the elevation of the adjacent synclinal valleys, creating what geologists call "inverted topography"(Figure 16a and 16b) Eventually, the resistant rock layer becomes a tough caprock that remains high above the surrounding terrain as the rocks within the adjacent anticlinal structures are removed It is important to emphasize that the summits of all of the ridges within the Appalachian Mountains are lower in elevation that of the easternmost edge of the Appalachian Plateau with the elevations of each more easterly ridge being generally lower than that to the west This relationship results in the rather unique situation of one going down into the Appalachian Mountains when approached from the west Figure 17 LE.G~-p ~ ~ CO~->E ~ ~~Ot lS ~ ~ ~ ~ Figure 16b SHA~ Ci::>A,._ 'f ~ ~f-JAL.E Because of the dominant northeast-southwest trending ridges and valleys, the stream pattern within the Valley and Ridge is trellis with the major streams cutting across the structures and tributaries draining the valleys (Figure 17) There is evidence that many, if not most, of the water gaps that cut across anticlinal structures follow vertical fault zones with the streams taking advantage of the zone of weakness The Great Valley Section: The Great Valley Section of the Valley and Ridge Province, commonly referred to as the Shenandoah Valley, extends eastward from the easternmost ridge of the Appalachian Mountain Section of the Valley and Ridge to the base of the Blue Ridge Mountains Rocks within the valley are nearly all Cambrian and Ordovician limestones Because of the water soluble nature of calcium carbonate, the rocks have been dissolved down to the mean level of the streams, resulting in a broad, flat valley The few low ridges observed within the valley are in large part due to the occasional non-carbonate rock layer that is a bit more resistant to erosion Although natural exposures of limestones are limited because of their solubility, numerous limestone rock outcrops can be seen in the fields Typical of Figure 18 regions underlain by limestones, the soils are thin and are composed almost entirely of the insoluble materials, clay minerals and quartz, that were originally contained within the limestones Also typical of areas underlain by limestones, the Great Valley shows extensive development of karst topography readily identified by the extensive number of sinkholes throughout the region (Figure 18) Because of their alkaline character, the soils are ideal for the growth of calcium-loving grasses which explains the widespread use of the land for the grazing of cattle The red to orange color of the soil is typical of areas underlain by limestone and is due to the fact that the Table insoluble materials released by the dissolution ORIENTATION WEST EAST of the limestones are coated by combinations PASSIVE ==;> ~ ACTIVE fORCES ofred iron oxides, Fe 20 3, and yellow iron oxyCOMPRESSION COMPRESSION hydroxides, FeO(OH) ENERGY MINIMAL MAXIMUM AVAILABILILTY Structurally, the valley is underlain APPALACHIAN PLATEAU VALLEY & RIDGE PROVINCE GEOGRAPHIC BLUE RIDGE by many northeast-southwest trending highPIEDMONT REGION MOUNTAINS LOW PLATEAU HIGH PLATEAU A~he~~~~~N GREAT VALLEY displacement thrust faults and highly deformed ]mllilDJOtnimmilll!D!m asymmetric and overturned folds which, FOLD ~ HIGHLY DEFORMED ~ GEOMETRY LOW AMPLITUDE MEDIUM AMPLITUDE ~ because of the limited exposures, are not PLASTIC FLOW HIGH AMPLITUDE HIGH AMPLITUDE SYMMETRICAL RECUMBENT SYMMETRICAL ASSYMETRICAL ASSYMETRICAL always easy to observe As one approaches the FAULTS MAY BE PRESENT DISPLACEMENTS: DISPLACEMENTS: DISPLACEMENTS: DISPLACEMENTS: DISPLACEMENTS: easternmost portion of the valley, the rocks (REVERSE) WITH VERY SMALL l's to 10's 10's to 100 s 100's to 1OOO's 1OOO's OF FEET ESTIMATED INTO show evidence of low-level metamorphism, Of FEET OF FEET Of rm TO MILES 1O's Of MILES ~ DISPLACEMENTS the result of being located closer to the original METAMORPHIC zone of deformation (Table 1) LOW MEDIUM HIGH FACIES ~ GRADE IGNEOUS ACTIVITY GRADE GRADE MAFIC TABULAR INTRUSIONS FELSIC PLUTONS 10 The carbonate rocks in the Great Valley are often so pure that they were both quarried and deep-mined Uses of the limestone products range from the flux-stone used in the iron and steel industry to remove silicate contaminants from blast furnaces, for the manufacture of cement, and to coat the walls of deep coal mines in order to reduce the amount of flammable coal dust in the atmosphere of the mine Throughout much of the northern Appalachian coal basin, it is used treat and inhibit the production of acid drainage The Blue Ridge Physiographic Province: The Blue Ridge Physiographic Province makes up the Blue Ridge Mountains Structurally complex, the Blue Ridge consists of highly deformed and metamorphosed pre-Cambrian and Cambrian rocks that have been intruded by basaltic and rhyolitic magmas The province is actually an eroded anticlinorium, the South Mountain Anticlinorium An anticlinorium is a broad regional anticlinal structure composed of lesser folds In the area of the trip, the province extends from South Mountain on the west to Catoctin Mountain on the east with Middletown Valley located in between Middletown Valley is floored by metabasalts and represents the core of the anticlinorium To the south, erosion within the valley has exposed the billion-year old basement gneisses that underlie the structure The basement rocks are of similar radiometric age beneath all of eastern North America! From central Ohio east to the Atlantic, the basement rocks are known as the Grenville Complex The Piedmont Physiographic Province: By definition, a piedmont is the plane or slope that exists at the base of a mountain In this case, it is the sloping surface that extends eastward from the base of the Blue Ridge Mountains and disappears under the recent sediments of the Coastal Plain The Piedmont Physiographic Province contains the most highly deformed rocks of the Appalachian region Because of their highly deformed character, the rocks within the Piedmont succumb quite readily to chemical weathering As a result, weathering over the past 100 million years has generated a thick regolith or saprolite that cover the rocks almost everywhere within the province Below the thick layer of weathered material are amphibolites, schists, and ultramafic rocks known collectively as the Baltimore Complex These rocks are interpreted to be oceanic crustal rocks that were shoved over the underlying younger rocks during a continent-continent collision that occurred 500 million years ago To the east, the Piedmont disappears under the Coastal Plain The contact between the Piedmont and the Coastal Plain is referred to as the "Fall Line" because of the small waterfalls that commonly occur where streams flow from the more resistant rocks of the Piedmont onto the more easily eroded recent sediments of the Coastal Plain (refer to Figure 19) One of the best locations to view the Fall Line is at Great Falls Park just northwest of Washington, D.C Here, the Potomac River has created a window to the Piedmont rocks which are otherwise covered by either saprolite or coastal plain sediments Because bedrock within most of the Piedmont is rarely exposed, our trip will end at Frederick, Maryland, on the westernmost edge of the Piedmont 11 The Coastal Plain Physiographic Province: Though not part of Appalachia, it is important to realize that the less consolidated materials of the coastal plain are, in reality, all the materials stripped from the Appalachians by erosion, brought east by rivers, and deposited along the eastern edge of the North American as part of its seaward extension, the continental shelf /1, The Origin and Structure of the Appalachians ~ elv F'"~l ~\o ~ x,fl- f ,,# -~T' ~ ~ ~,.) -vl ~~ ~v b~~~ (, ~~ \,,~ ,,~ In order to get the most out of any field v'o ~ l ~v l -:o& trip, it is essential to have a basic picture of 7~ ,Q-' the combined structure, stratigraphy, and ~ i l v~J ,96 l ~ O erosional history of the area Understanding Y'(J,~f"-v ,,{i'~ ,,/· J how the structures within the area formed, although not essential, will add significantly ~ to your overall understanding of the geology In order to provide you with such an understanding, I must go beyond what ~v,ue: you would learn in an introductory geology 5'.t-.1~1i::; course In the case of the Appalachians, it t'Oc~ 7U:{;.A.~TA, SA~I£ 5.AN.~'l'-l!a requires introducing you to a concept that describes the fate of sedimentary rocks that \-\\St'f.Sr a.Ev~:! 1µ A;~Arl-\1At-J'i are involved in a major mountain building episode The fact that you chose to go on this trip indicates that you are interested ~~.::5£.t\ LEV E.'EAS,WE ST in understanding the geology of the ~.1fPl-llC7'iZ6F1 L : Figure 19 ~1 e~Hl:l l½' o.- A-n-AL.Ac111 A~~ Appalachians and would probably appreciate ~~ rl2-0"'1T a more in-depth understanding of what you will see To illustrate how the structures formed, I have prepared two figures, Figure 19 which is a block diagram depicting the present topography along with the various physiographic provinces and the subsurface structures and Figure 20 (next page) that consists of five drawings that sequentially illustrate the evolution of the Appalachian structures The great mountains of the world were created by the collision of continents A modem example, the Himalaya, are the result of the collision of India and Asia beginning about 45 million years ago, a collision that is not yet over as indicated by the frequency of earthquakes throughout the region combined with the fact that the Himalaya are still rising The Himalaya, and all of the great mountains of the world including the Appalachians, are examples offoldbelt mountains, the name referring to the fact that a major portion of the mountain range ,v \~ ~ -~ c=:: t 12 ... Appalachian Mountain Section SECTIONS OF THE APPALACHIAN PLATEAU PROVINCE A8 CDEFG- Mohawk Section Glaciated Allegheny Plateaus Catskill Mountains Allegheny Mountains Unglaciated Allegheny Plateaus... Allegheny Plateaus Cumberland Mountains Cumberland Plateau )( L ~ _ 100 100 200 200 Appalachian Low Plateau: The Appalachian Low Plateau is the westernmost of the Appalachian Plateaus Physiographic provinces... of the Appalachian Plateaus An important economic aspect of the Appalachian Plateau are the mineable coal deposits that are located within its boundaries with the coal beds being contained within